ocular pain and the trigeminal nerve

Question 1

what are the 2 classes of somatic sensory receptor ?

Answer 1

1. free nerve endings
   . pain or temperature sensitive
2. specialised mechanoreceptors
   . tactile receptors ( touch , pressure )
   . proprioceptors ( joint position )

Question 2

how is the somatic sensory innervation of all the face and eye controlled ?

Answer 2

via trigeminal (5th ) cranial nerve which mediates tactile , thermal and pain sensation .

Question 3

where are the somatic sensory receptors in the eye ?

Answer 3

. anterior corneal surface/epithelium
. inside the eye ball : uvea = comprises several tissue in the eyeball specifically the choroid , ciliary body and iris
.orbital contents and eyelids + conjunctiva

Question 4

what is the significance of somatic sensory innervation ?

Answer 4

. it is about ocular defence
when foreign bodies land on surface of your cornea , so the sensory receptors alert you to the presence of those via the corneal eyeblink reflex
-Ocular pain

Question 5

what is the significance of ocular pain ?

Answer 5

alerts the PX to damage or disease

Question 6

what are major branches of trigeminal ( 5th ) cranial nerve ?

Answer 6

. ophthalmic ( V1 ) - takes sensory information from anterior corneal epithelium , uvea and upper eyelid and conjunctiva and lacrimal gland

. maxillary ( V2) - branches of maxillary nerve that supply lower eyelid and conjunctiva

. mandibular ( V3) - lower jaw + teeth

Question 7

what are the 3 main ophthalmic ( v1 ) nerve branches ?

Answer 7

1. nasociliary nerve : takes the medial orbital path
   . long and short ciliary branches - take sensory information from inside the eye and the surface of the cornea
   . nasal branches : take sensory information from inside the nasal cavity
   . infra-trochlear :
   medial upper eyelid + conjunctiva
2. lacrimal nerve :takes the lateral orbital path
   takes sensory information form the lateral upper eyelid + conjunctiva and lacrimal gland

3.frontal nerve : runs through upper orbital path
takes sensory information from the skin of your forehead and scalp

Question 8

how the 3 main ophthalmic ( v1 ) nerve branches exit the orbit ?

Answer 8

exit the orbital via superior orbital fissure as V1

Question 9

what do all small nerves and the 3 main nerve branches contain ?

Answer 9

. peripheral axons of things which are trigeminal ganglion cells
. each nerve branches ophthalmic , maxillary and mandibular have axons whose cell bodies sit in that ganglion and come together to form 5th trigeminal nerve root that goes into the brain stem and sends sensory information to various nuclei within the brain stem

Question 10

where are the trigeminal ganglion cells located ?

Answer 10

trigeminal ganglion cell bodies sit in a bag just outside your brain stem

Question 11

what does the surface of cornea contain ?

Answer 11

contains very high density of these different types of sensory receptor

Question 12

where are the receptors in the cornea and ( uvea ) present ?

Answer 12

. receptors are present in the anterior epithelium

. their axons pass via bowman’s membrane to

• run radially through the stroma to leave in all directions at the limbus and enter the supra-choroid just below the sclera
• and join axons coming from receptors in the uveal tract and those axons also leave via short and long ciliary nerves at back of your eyeball

Question 13

what are the two types of receptors in the axons in the long and short ciliary nerves leaving the back of the eye ?

Answer 13

. 50% specialized mechanoreceptors which signal touch and pressure
- they produce phasic ( fast - adapting ) responses to mechanical stimulation only

. 50% poly-modal nociceptors from free nerve - endings ( pain )
- they are tonic / long lasting ( slowly adapting ) responses to different noxious mechanical , thermal and chemical stimuli

Question 14

what does poly-modal mean ?

Answer 14

pain receptors might respond to noxious severe mechanical deformations to cuts or stabs
. respond to severe thermal stimuli

Question 15

what is a simple mechanism for ocular defence ?

Answer 15

corneal - eyeblink reflex

Question 16

what are the two types of corneal - eyeblink reflex ?

Answer 16

1. sensory limb : light corneal touch/dryness - mediated by 5th trigeminal nerve
   . activates receptors on corneal surface
   .those receptors have axons travelling via long ciliary , nasociliary and ophthalmic nerves
   . then axons travel via 5 th nerve root to trigeminal brainstem nuclei which then signal to nuclei which signal to orbicularis oculi muscle which is blinking muscle
2. motor limb : close eyelids/blink - mediated by 7th facial nerve

. somatic motor neurons in main facial nucleus
. axons run in facial nerve , then travel via temporal and zygomatic branches to forcefully contract orbicularis oculi

Question 17

what are the two major parts of the orbicularis oculi muscle ?

Answer 17

palpebral - part that runs through the eyelids themselves
contract during gentle blinking
orbital - runs through the orbital rim
forceful contraction used during ocular defence

Question 18

what kind of tissue damage do pain stimuli and nociceptors respond to ?

Answer 18

respond to tissue damage caused by combination of :
. mechanical deformation ( pinch , cut )

. temperature extremes ( heat ,cold)

. chemical irritants ( e.g. acid , histamines )

Question 19

how are pain stimuli detected ?

Answer 19

by 2 major nociceptors

Question 20

what are the 2 major nociceptors classes ?

Answer 20

1. c- fibres : 2/3 of receptors in cornea
   . small diameter , unmyelinated axons thus slow rate conduction
   . respond to dull , aching , throbbing ( cold ) and stinging pains
   . usually polymodal respond to any/all of the 3 types of tissue damage
   . activated by this particular chemical capsaicin
   . respond to histamine also
2. A𝛿-fibres : small diameter axons , some myelin
   . respond to mechano-thermal receptors , not chemical
   . respond to sharp , stabbing , hot pain

Question 21

what is the difference in pain localisation between skin tissues and visceral seep tissues = eye ?

Answer 21

1. cutaneous ( skin ) tissue
   able to localise pain accurately in the surface of your skin this is sue to
   . free nerve endings in skin that belong to a single axon
   . free nerve ending is only responsive to stimuli from a very small region
2. visceral ( deep ) tissues = eye ?

.many , widespread endings per axon
. large receptive field and overlapping
. poor localisation of noxious stimuli
. can be referred

Question 22

what does referred pain mean ?

Answer 22

pain perceived at a location that is not the actual injury site
e.g. pain down left arm with heart attack this is because those axons have branches that go to both of those tissues

Question 23

what are some causes of ocular pain ?

Answer 23

1. naturally - occurring or in the eye
   . dry eye and corneal scratches ( irritation )
   . uveitis ( infection ) , glaucoma ( raised IOP )
   . tumors ( e.g. melanoma ) formed by melanocytes
2. naturally - occuring oustide the eye
   . orbital blow out fracture ( trauma )
   . conjunctiva and orbital cellulitis ( infection )
   . optic neuritis ( inflammation of optic nerve )
   . herpes zoster ophthalmicus ( inflammation of V1 )
3. latrogenic ( caused by health care professionals )

. eye drops ( e.g. acidic vehicle for atropine )
. laser surgery ( to repair retinal tears )

Question 24

what is the condition anterior uveitis ?

Answer 24

. inflammation of uveal tract
so iris and ciliary body and choroid will be inflamed
. this causes deep dull pain which caused by the release of histamine
. can also be caused by autoimmune, microbial
. T-cell mediated
. spreads from uvea and gets into blood vessels
. anterior iridial circle also becomes inflamed
. detected by circum - limbal flush
. treated by steriods which causes immune suppression and anti inflammatory

Question 25

what is the condition of orbital blow-out fracture ?

Answer 25

. caused by trauma from object larger than the diameter of the orbital rim strikes the orbital rim with large amount of force
. this cause pressure waves inside the orbital cavity and those pressure waves cause the bone that forms the main floor of orbital wall to fracture and that bone is the maxilla
. contents of orbit drop into fracture , maxillary sinus
. causes sunken eye ( enophthalmos ) , double vision ( diplopia ) , restricted up and down gaze trapped inferior eye muscles

Question 26

what is the condition right orbital cellulitis ?

Answer 26

. common cold , leading to chronic sinusitis , then invasion of bacterial infection from air sinus in ethmoid through the bone , into orbit and then outside the eye
. symptoms : red and swollen eyelids , ocular pain , especially on eye movement

. back of orbit there are holes , those holes lead to brain and those bacteria get to brain and cause encephalitis and cause of encephalitis is death

Question 27

what is the condition herpes zoster ophthalmicus ( shingles ) ?

Answer 27

. painful rash ( vestibular eruptions ) affecting v1 nerve territory : chicken pox virus dormant within the nerves but reactivated by immune suppression especially in the elderly

Question 28

how can you get referred ocular pain ?

Answer 28

you can get that in the eye because you got damage somewhere else
. this happens when membranes covering the brain by inflammation ( meningitis ) or by compression ( brain tumours ) , this leads to eye pain
. if arteries get pathologies in them for example aneurysm or arthero-sclerosis you can get eye pain

Question 29

what is the reason for referred ocular pain ?

Answer 29

. this is due to pain receptors located in the membranes and walls of large arteries
. they have axon branches that also run in the ophthalmic and ciliary nerves into the eye
. Px experiences ocular pain , because it is as if the pain receptors in the cornea and uvea are also being stimulate

Question 30

how is ocular referred pain mediated ?

Answer 30

mediated by axons known as trigemino vascular

Question 31

what are endorphins ?

Answer 31

they are opiates just like heroin and morphine and those neurons are release these opiates into brain and spinal cord which travel to different parts of your tissues and lower the pain sensation

Question 32

what are the mechanisms for reducing pain transmission ?

Answer 32

. opiates
which activates endogenous endrophin receptors

. local control via inhibitory interneurons
. activated by fast- conducting mechanorecpetors
this activates particular group of neurons that release opiates which sit in layer two of the spinal cord and equivalent trigeminal nucleus
. inhibits/blocks layer V neurons receiving pain signals from sending them to the brain
. this is known as gate-theory
. trans - cutaneous electrical nerve stimulation

Question 33

how does the gate theory of pain work ?

Answer 33

so when you cut your finger

1. rub it better to stimulate type 1,2 mechanoreceptors
2. activated inhibitory interneuron
3. which inhibits projection neuron
4. blocks transmission of nociceptive signals to the brain

Question 34

what is tens machine ?

Answer 34

.can put it in area of pain , this sends electrical impulses down and it stimulates these mechanoreceptors to activates interneurons and reduce pain transmission

. can be used for pain-relief in herpes zoster by stimulating superficial , cutaneous ( infra-trochlear ) branches of the ophthalmic nerve

Question 35

Which of the nerves carries sensory information from the cornea?

Answer 35

Naso-ciliary